Aromatic metallacycles represent a fundamentally new class of aromatic compounds in which metal d orbitals participate fully with carbon and heteroatom p orbitals in ring a-bonding. Proposed herein is a general synthetic route to aromatic metallacycles, which promises to allow the rational and systematic synthesis of a large family of these novel and exciting molecules. The proposed route utilizes C-H bond activation in (pentadienyl)metal precursors as the key ring-forming step. The pentadienyl-based synthetic approach has already been show to produce in high yield the six-membered aromatic iridacycle, "iridabenzene." The same methodology will now be extended to other late transition metals with the goal of synthesizing osmabenzene, rhodabenzene, and platinabenzene complexes. In addition, heteropentadienide reagents including oxa-, thia, aza-, and phosphapentadienides will be used as building blocks in the synthesis of heteroatom-containing aromatic metallacycles. Specific synthetic targets include metallafurin, metallathiophene, metallapyrrole, metallapyrylium, metallathiopyrylium, metallapyridine, and metallaphosphabenzene. The physical properties of the new metallacycles, particularly their solid state structure, magnetic behavior, and solution phase spectroscopy, will be probed. These physical data will allow assessment of their "aromatic character" and enable direct comparison with better-known organic and heteroorganic analogues. The reactivity of the new metallacycles toward dienophiles, electrophiles, and nucleophiles will be investigated. While some of these reactions are expected to mimic those of conventional arenas, others will differ sharply due to the powerful influence of the transition metal center. From this study, new guidelines for what constitutes "aromatic behavior" in metallacycles will emerge. Finally, Fenske-Hall molecular orbital calculations will be carried out in conjunction with the experimental work. These calculations will provide a better picture of the bonding in these novel molecules and a better understanding of the electronic origin of their aromaticity.